What’s the right way to do a patch test and why does it matter for this product? — Toning shampoo light brown 40ml V21 victoria beauty

Even though this is a non-oxidative, semi-permanent shampoo, the direct dyes and other ingredients can still trigger contact allergy in sensitive people. To test, mix a tiny amount of the product as you would for use, apply a pea-sized dab behind the ear or on the inner forearm, and leave it uncovered for 48 hours. Wash the area only if irritation appears. Watch for redness, itching, swelling, burning, or any rash — these signal a possible allergic reaction. If nothing happens after two days, the risk of a reaction during full application is low. Do this test before every first use of a new batch or if you’ve had a tattoo, henna, or previous hair dye reaction since your last use, because sensitisation can develop over time. If you have a known sensitivity to cosmetic colourants, consult a dermatologist before proceeding.